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<title>Debugging with GDB: Arrays</title>

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<a name="Arrays"></a>
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<a name="Artificial-Arrays"></a>
<h3 class="section">10.4 Artificial Arrays</h3>

<a name="index-artificial-array"></a>
<a name="index-arrays"></a>
<a name="index-_0040_002c-referencing-memory-as-an-array"></a>
<p>It is often useful to print out several successive objects of the
same type in memory; a section of an array, or an array of
dynamically determined size for which only a pointer exists in the
program.
</p>
<p>You can do this by referring to a contiguous span of memory as an
<em>artificial array</em>, using the binary operator &lsquo;<samp>@</samp>&rsquo;.  The left
operand of &lsquo;<samp>@</samp>&rsquo; should be the first element of the desired array
and be an individual object.  The right operand should be the desired length
of the array.  The result is an array value whose elements are all of
the type of the left argument.  The first element is actually the left
argument; the second element comes from bytes of memory immediately
following those that hold the first element, and so on.  Here is an
example.  If a program says
</p>
<div class="smallexample">
<pre class="smallexample">int *array = (int *) malloc (len * sizeof (int));
</pre></div>

<p>you can print the contents of <code>array</code> with
</p>
<div class="smallexample">
<pre class="smallexample">p *array@len
</pre></div>

<p>The left operand of &lsquo;<samp>@</samp>&rsquo; must reside in memory.  Array values made
with &lsquo;<samp>@</samp>&rsquo; in this way behave just like other arrays in terms of
subscripting, and are coerced to pointers when used in expressions.
Artificial arrays most often appear in expressions via the value history
(see <a href="Value-History.html#Value-History">Value History</a>), after printing one out.
</p>
<p>Another way to create an artificial array is to use a cast.
This re-interprets a value as if it were an array.
The value need not be in memory:
</p><div class="smallexample">
<pre class="smallexample">(gdb) p/x (short[2])0x12345678
$1 = {0x1234, 0x5678}
</pre></div>

<p>As a convenience, if you leave the array length out (as in
&lsquo;<samp>(<var>type</var>[])<var>value</var></samp>&rsquo;) <small>GDB</small> calculates the size to fill
the value (as &lsquo;<samp>sizeof(<var>value</var>)/sizeof(<var>type</var>)</samp>&rsquo;:
</p><div class="smallexample">
<pre class="smallexample">(gdb) p/x (short[])0x12345678
$2 = {0x1234, 0x5678}
</pre></div>

<p>Sometimes the artificial array mechanism is not quite enough; in
moderately complex data structures, the elements of interest may not
actually be adjacent&mdash;for example, if you are interested in the values
of pointers in an array.  One useful work-around in this situation is
to use a convenience variable (see <a href="Convenience-Vars.html#Convenience-Vars">Convenience
Variables</a>) as a counter in an expression that prints the first
interesting value, and then repeat that expression via <tt class="key">RET</tt>.  For
instance, suppose you have an array <code>dtab</code> of pointers to
structures, and you are interested in the values of a field <code>fv</code>
in each structure.  Here is an example of what you might type:
</p>
<div class="smallexample">
<pre class="smallexample">set $i = 0
p dtab[$i++]-&gt;fv
<span class="key">RET</span>
<span class="key">RET</span>
&hellip;
</pre></div>

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